Since the main measurements for bike fit are relatively simple (standover, torso,forearm, reach etc), you ought to be able to fit a stick man on a bike and see problems such as not enough bend in the arms for a normal riding position? It might be easier to pick up issues than looking at a real person?

You can ask a real person questions, and opinions..

You are in a tech job I guess , so don't have history of one of those jobs that sells to real bike customers..
thats OK those jobs don't pay like IT jobs do.

People are not just data,in case you wondered ..


Plus for the most part the customer shows up at the end of the line ,
walks into the shop and wants a built up bike and to test ride it ,
and the sales people are left to switch out stems and put them on a medium or a smaller or larger size frame as their height dictates..
[been there on the shop floor trying to find what we had that would work ]

the CAD CAM stuff is all sorted out by the companies a long time ago,

I suspect that a computer model could help if you gave it enough input data fields, but I doubt that such a procedure would be cost-effective for anyone besides top racers. In order for a stick-man approximation to be worthwhile, you would have to define things like "enough bend in the arms", which is going to vary based on a whole lot of biological factors.

Even the very basic data, like bone length and weight distribution, would be difficult to measure precisely without specialized equipment.

I asked this question not long ago and got an inexplicably hostile reception in the forum. Being of an R&D background I never understood this attitude. If we all did only what was tried and true we'd all still be riding horses, not bikes. Big tech companies fit their products this way, ie, airplanes, cars, etc. The problem is the number of man hours required would make it cost prohibitive to do it for a single person unless there was a lot of money involved such as racing the Tour de France.

A mechanical engineer acquaintance explained that bike frames need to be analyzed using finite element analysis and that it is not trivial. There is software out there to do it, but there's a good reason why custom bikes are expensive: even experienced craftsmen who can eyeball the general shape and materials of a frame sometimes get unhappy results like flex, etc. Unless you have megabucks like Trek, Specialized, etc. there's a lot more art than science in this craft to make a bike that not only fits right but "feels" right to the user.

https://en.wikipedia.org/wiki/Structural_analysis

I suspect we are thinking of different sectors, the full custom frame-set being made for and with
a knowledgeable Pro level rider , I expect the team members of the big Pro teams get that kind of service
from their team's sponsoring bike Company.

versus the bulk of the market , people who walk in to a Retail shop and want to ride a bike,
That is already on the shop Floor to see ..

BikeCad does a reasonable job of designing a frame given a set of measurements. Then it will show you the rider on the frame it designed. Of course, there are a lot of fit calculators out there. This isn't quite the same thing as fitting a rider to a stock frame, but it's not that hard to back out the info you want.


I don't see any reason to do this other than for a fully optimized carbon frame that is made to be as light as possible. People do not understand what makes a bike that feels right to most people. If there ever was a touring bike that was designed using finite element, it was for marketing purposes.

I wasn't thinking of designing the perfect bike but showing how basic frames fit basic dimensions. Just about every manufacturer gives you a geometry chart and if you combine that with inseam,torso,forearm, reach, thigh, lower leg, sternal notch and height it may give a good indication of the best frame size without the fine tuning. What made be think of it is this thread (below). It is hard to figure somethings but a model might reveal what is wrong. I think in my case my hands prop me up like legs on a stool, with no bend. Most people on that thread think my bike is too big but the seat is almost as high as I can get it and I haven't got a lot of spare stand over. What I'm saying is a simple model may show the dynamics of your particular body type (long torso short arms/short torso long arms etc).
https://www.bikeforums.net/showthread...0#post11918930

We eagerly await your software graphics solution on this..
you are going to write one aren't you?
or is this for the 'You Guys Should' study group to tackle?

back in the day .. we used scale drawings on paper to do those sort of things
You .. could make a You Stick figure out of card stock
and pins for joints of knees hips and elbows
and make a bike stick on a piece of paper too, and try it out.

Off the peg frames may not work for you, hire a custom frame , a short size with a tall head tube and a long seat post. to get reach the same and gain standover .. sloping top tube, or as I saw in some Dutch bike companies years ago, a J bent top tube, that like a ski tip curved up to meet the steerer tube higher

Of course with threadless steerer tubes they start out as 300mm long , just don't cut it down, add more spacers underneath your stem..
On my Trekking bike I use the extra steerer tube for a 2nd stem , it has the handle bar bag on it.

Your "perfect" position is the same no matter what frame you are on. That's why I suggested the one cad program that effectively meets your stated requirements

Bruce Gordon Makes great touring bikes and has been using the data inputs you mentioned
to build custom or select a stock size to suit your needs . for over 30 years..

write or call him...

You walk into the custom frame builder's shop and tell him you want a racer, a mtb, a touring bike. "Step into this body scanner we borrowed from TSA." he instructs. Couple minutes later the computer has an exact copy of your dimensions as well as design details for 3 custom frames. A month and $20,000 later, you're delivered 3 bicycles that fit you perfectly. Any conceived imperfection will be your imagination. Get over it.

Technically, this scene is already possible. When is the question.

Agreed. I was merely pointing what could be done to get proper analysis, not that it necessarily makes sense for most of us. When asked about the need for finite element analysis the engineer said it is a common technique for new structures when the stresses are not well known and the number of products or cost of the item (ie, a bridge) justifies it. It is just a tool, not a substitute for modeling, prototyping, regression testing and other design methodologies. For a well known item it may not be appropriate or needed. No one trusts software alone anyway. In the case of bikes, the big boys sponsor racing teams to get a lot of useful real world data to go with their fancy computer design work.

The technical benefit of all this work is wasted on a single person's custom bike -- too costly for the benefit when we have the accumulated empirical knowledge of a 100 years of good bikes, especially for steel frames. Unless working on something unusual or composite materials, I'd stick to trial and error and the experience of a custom builder who has got a few years under the belt. That's part of what you are paying for and cheaper in the long run.

Actually, this is an interesting proposal. How would you approach doing this in a reasonable amount of man hours?

It wouldn't be that hard to do for someone familiar with AutoCAD or similar. Somewhat tricky to locate the pivot points and lengths. Easiest to locate the sitbones on a current bike and measure from there. The location of joints and their movement is a little complicated, but shouldn't be too hard to get good approximations. Need a good assistant who's somewhat familiar with these issues to do the measuring. Then it's just a matter of cadding it out. Should be able to do it in a couple of hours after getting the measurements. Do one with lower body, pedals aligned with seat tube to get saddle height, save and copy, modify for saddle set-back, save and copy, add torso and arms, get stem length.

The bigger question is "what's it supposed to look like?" That varies with the person, their experience, their training state, and what they're used to, if anything. That's why people go to a good bike fitter. That process is quicker than CAD and, if the fitter is any good (an important question), gives reliable results.

Rather than try to solve these sorts of problems oneself, the simple way is to go to a person who knows more about it than you do. We can't see and touch you, big disadvantage.

A Motion Capture suit , like the movies use for actors to go thru the motions , which are processed
by the computer special effects technicions , perhaps?

How many hundreds of thousands of dollars do you have to devote to developing this ?


back to the Q&A ... whats wrong with your present bike fit, and what changes have you tried already?

Actually, you can rent. People in the Computer Animation business do it all the time. Anybody have friends at Pixar?

Given that manufacturers provide basic geometry data and you can provide your own basic biometrics, could the two be combined to analyse a likely best fit, based on mechanics (lengths and angles) or empirical data? Would the "sweet spot" be determinable?
A fitting session on a set up device ought to help choose but presuming someone hasn't done that and not everyone can walk into a shop and find a suitable bike in their size. In my experience even supposed experts can't always tell a good fit by looking (having experienced a contrary opinion).

The sweet spot is personal. Two people with identical measurements might like slightly different positions. For example, some people like riding down on the drops, with their back a little arched, others like a more upright position. In my case, as I have aged I prefer a more upright position. So even the same person can change over time. While doing the exercise of taking a given geometry and seeing if your measurements sort of fit is useful, riding the bike is needed for that extra subjective factor.

Many people go in the opposite direction when they become experienced riders, taking their measurements and style as a given and asking for a bike that fits them rather than trying to fit something off-the-rack. That's why we sort of half-joke about the motion capture suit. It actually makes sense!

I realise that no two people are exactly alike, but I would have thought we are alike enough that two people with short legs and long torsos are likely require a different set up to two people with long legs and short torso? I was thinking it would be interesting to put a figure on a frame and then change torso, arms etc to see if comfort/discomfort can be observed within the adjustable parameters for a frame size. Ultimately I was wondering if it could be used as a buying guide.

OK it would be interesting, that can be agreed to, but apparently no one on the BF, so far,
is aware of a Program written to do that.
If there were, how much would you be willing to pay to get a copy?

take data off what you ride already, analyze that information , ask yourself what you would like to change.

I have a bike great , but I would like a lower seat tube angle to create more setback
to take the weight off my arms , that change has ramifications on a number of other relationships
of the other angles and tube lengths ..

looking at a bunch of 73 degree frames .. when a 68 is desired, won't get them to change.

I have to get something built custom, probably ..

most people can just put a different stem on the bike on the floor.. and be happy enough.

threadless , open face stems make that So Easy.